Apparatus of extruding a plastic pipe under control of the wall thickness of the extruded plastic pipe

ABSTRACT

A pipe of thermoplastic polymer, e.g. polyvinylchloride is extruded by an extruder comprising an extrusion die (2) having a moulding end part (2a) with heating elements (11) located about said end part. The wall thickness of the pipe is controlled in order to obtain a pipe of a substantially even wall thickness. For said control the wall thickness of the extruded pipe is measured at 360 points equally distributed over the circumference and moreover the average is determined in eight measuring sectors comprising 45 measuring points per sector. The average of the measuring values of one measuring sector is compared with the circumferential average of the 360 points, the deviation of the desired wall thickness in said one sector being counter balanced by the action of the heating elements. In a preferred embodiment a polyvinylchloride pipe obtained by means of a two screw extruder presents an even wall thickness over the circumference.

This application is a divisional of application Ser. No. 885,073, filedJuly 14, 1986, now U.S. Pat. No. 4,749,531, which is a continuation ofapplication Ser. No. 686,535, filed Dec. 26, 1984, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention The invention relates to a method of extrudinga rigid plastic pipe of a thermoplastic polymer and controlling the wallthickness of the extruded pipe by means of a screw extruder providedwith an extrusion die comprising a moulding end part, the temperature ineach of several temperature-controlled sectors disposed about themoulding end part of the extrusion die being controlled as a function ofthe measurements of the wall thickness at a number of measuring points,each located in measuring sectors which lie in the axial extension ofthe temperature-controlled sectors and more particularly to a method ofextruding a rigid polyvinylchloride pipe in this way.

By the expression a rigid plastic pipe is meant a pipe being flexible ornot having such a wall thickness that this pipe maintains substantiallyits shape during storage at a flat support and does not collapse.

2. Description of the Prior Art

A method of this kind is known from Dutch patent application No. 72 09444. According to this known method, the wall thickness of the pipe, ata point where an extruded plastic pipe has assumed solid form, ismeasured by means of a backscatter-type sensing device rotating aboutthe pipe, whereupon the signals emanating from the sensing device aretransmitted to a control unit. From this control unit, signals are sentto heat-supply or heat-removal elements provided in each of a number ofindividual temperature-control sectors disposed about the moulding endpart of the extrusion die, thus effecting the temperature in each ofsaid temperature-control sectors. As the flow rate of the melted plasticin the moulding end part of the extrusion die varies in accordance withthe temperature, the thickness of the plastic pipe will consequentlychange in such a temperature-control sector area.

Said known method has the drawback that it is difficult to obtain a pipehaving a thickness which is very even over the entire circumference ofthe pipe notwithstanding a regular distribution of the heat supply orheat removal elements about the circumference of the pipe.

This disadvantage is particularly important in pipes obtained byextruders having two screws which pipes comprise principally twoopposite thickened wall parts extending in a longitudinal direction ofthe pipe.

In such cases these disadvantages cannot be removed by a bettercentering of the core in relation to the wall of the moulding end partof the extrusion die.

SUMMARY OF THE INVENTION

The invention aims to overcome the abovementioned disadvantages.

This object is attained according to the invention in that the wallthickness is measured in at least one measuring sector at several pointscircumferentially located next to one another the average of thesemeasurement values is determined and compared with the average of thevalues measured in all measuring sectors, and the temperature in thetemperature-controlled sector located in the axial extension of the onemeasuring sector aforementioned is controlled as a function of thedifference observed so as to remove the difference measured.

More particularly the invention relates to a method of extruding a rigidpolyvinylchloride pipe and controlling the wall thickness of theextruded pipe, the temperature in each of several temperature-controlledsectors disposed about the moulding end part of the extrusion die of ascrew extruder being controlled as a function of the measurements of thewall thickness at a number of measuring points, each located inmeasuring sectors, which lie in the axial extension of thetemperature-controlled sectors, wherein in applying an extrudercomprising at least two screws the wall thickness is measured in atleast one measuring sector at several points circumferentially locatednext to another, the average of these measurement values is determinedand compared with the average of the values measured in all measuringsectors, and the temperature in the temperature-controlled sectorlocated in the axial extension of the one measuring sectoraforementioned is controlled as a function of the difference observed soas to remove the difference measured.

Measuring the wall thickness at several points circumferentially locatednext to each other and comparing these values with an average ofmeasurement values is known per se for plastic foil tubes.

However in this process the wall thickness differences are adjusted bymeans of cooling elements provided outside the moulding end part of anextrusion die.

Acting upon heating elements in a moulding end part of an extruder diefor foil tube is possible but causes undesired side effects in the foiltube. In extruding foil tube the whole wall thickness of the foil tubewill immediately obtain the temperature given by the heat supply or heatremoval elements. This leads to more stretch in the hot regions incomparison with less hot regions during the inflation of the foil tubeand thus the foil tube will have a smaller wall thickness in theseregions in maintaining the other parameters at a constant value, such asoutlet velocity.

In extruding a rigid pipe the heat supply elements will only increasethe temperature of a superficial layer of the pipe permitting a morerapid extrusion of the pipe and thus increasing the pipe wall thicknessapart from maintaining constant parameters such as the outlet velocity.

More particularly it should also be noted that in the foil tube the wallthickness is adjusted by cooling behind the moulding end part of theextrusion die being less accurate than the compensation of a too smallwall thickness of a plastic pipe by supplying heat to the moulding endpart of the extrusion die.

As in the method of the invention wall thickness deviations from theaverage wall thickness are compensated, the result is a very uniformwall thickness over the circumference of the pipe.

In order to improve the uniformity of the wall thickness, the wallthickness is measured in all measuring sectors at several pointscircumferentially located next to each other.

Preferably, the wall thickness is measured at 360 points equallydistributed over the circumference, and the average is determined ineight measuring sectors at 45 measuring points per sector.

In this manner, a particularly even wall thickness is obtained.

Preferaby, the various signal processing operations are performed bymicroprocessors.

This enables differences in wall thickness to be eliminated very rapidlyand in a simple manner, and consequently a plastic pipe to be made whosewall thickness over the circumference of the pipe is quite uniform overan unlimited length.

The invention also relates to an apparatus for executing the process ofthe invention comprising a screw extruder with an extrusion diecomprising a moulding end part and disposed about said moulding end parttemperature-control elements being active in temperature-controlsectors, at least one wall-thickness measuring device for measuring thewall thickness of an extruded pipe, said measuring device being capableof performing measurements in all measuring sectors located in the axialextension of the temperature control sectors, wherein at least onemeasuring sector, a wall-thickness measuring device can perform severalmeasurements and transmit such measurement data to a signal processingunit wherein said measurement data are averaged and the data measured inall measuring sectors can also jointly be transmitted to a signalprocessing unit wherein said measurement data are averaged, a comparatorelement being provided for comparing the two values of themeasurement-value averaging operation, said comparator element beingcapable of acting upon a temperature-control device for controlling thetemperature of the heating elements in a temperature-control sectorlocated in the axial extension of the one measuring sectoraforementioned.

Such an apparatus has a very simple and inexpensive constructions, andcan therefore be easily put to use.

Preferably the apparatus comprises a screw extruder having at least twoscrews.

The invention also relates to a plastic pipe of a thermoplastic polymer,particularly a polyvinylchloride pipe, obtained by means of an apparatusaccording to the invention.

More particularly the invention relates to a polyvinylchloride pipeobtained by means of a screw extruder having at least two screws,wherein the polyvinylchloride pipe is substantially free from thickenedwall zones being substantially opposite each other.

At last the invention also relates to a measuring device and temperaturecontrolling device suitable for use in an apparatus according to theinvention.

Other claims and many of the attendant advantages will be more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description and considered in connection with theaccompanying drawings in which like reference symbols designate likeparts throughout the figures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic side view of a plastic-tube extruder comprisingan apparatus for controlling the wall thickness according to theinvention;

FIG. 2 is a front view of the extrusion die as used with an extruderaccording to FIG. 1;

FIG. 3 shows a wall-thickness profile as measured over the circumferenceof a tube;

FIG. 4 represents the average wall-thickness profile per measuringsector.

FIG. 5 represents a diagrammatic cross-section of another extrusion dieas used with an extruder according to the invention, and

FIG. 6a a cross section of a plastic pipe obtained by an extrudercomprising two screws according to the prior art; and

FIG. 6b a cross section of the same plastic pipe obtained according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an installation for manufacturing plastic pipe comprisingan extruder with an extrusion die 2 comprising a moulding end part 2afrom which the pipe issues. Said moulding end part 2a and the extrusiondie 2 comprise a core 14 and a wall 16 bounding an annular extrusionslit 15. The extruder extrudes through the extrusion die and at last themoulding end part of said extrusion die. The pipe of polyvinylchloride3, passes after extrusion successively through a calibrating device 4, acooling arrangement 5 and a drawbench 6.

The installation furthermore comprises a sensing device 7 rotatableabout the pipe and enabling the pipe wall thickness to be measured bymeans of ultrasonic waves at several points distributed over thecircumference.

Provided around moulding end part of the extrusion die 2 are heatingelements 11 which enable the temperature to be influenced in a number oftemperature-control sectors 12a-12h around the moulding end part of theextrusion die. The various heating elements 11 are connected to acontrol unit 8 for controlling the temperature in each of thetemperature-control sectors 12a-12h.

In addition, the installation comprises a first signal processing unit 9the input of which is connected to a sensing device 7 and one output ofwhich is connected to the drawbench 6, another output of which beingconnected to the input of the second signal processing unit 10. Theoutput of the second signal processing unit 10 is connected to thetemperature-control unit 8.

The apparatus operates in the following manner.

During extrusion of a rigid thermoplastic pipe 3, e.g. PVC pipe, thesensing device 7 rotates about the pipe and continually orintermittently measures the pipe wall thickness at a number of pointsdistributed over the pipe circumference. Plotting the values measuredover the circumference of the pipe in a diagram results in the diagramof FIG. 3 which shows diagrammatically the wall-thickness profile of thepipe. At a certain point, the wall thickness is indicated by S.

The signals coming from the sensing device 7 are transmitted to thefirst signal processing unit 9 wherein the minimum value of the wallthickness is determined and compared with the minimum wall thicknessdesired. The difference between these values is used to set the speed ofthe drawbench. Thus, the overall thickness of the pipe 3 is beingcontrolled, an increase of the drawbench speed generally resulting in athinner pipe and a decrease of the speed generally resulting in athicker pipe. Signals corresponding to wall thickness as measured at thevarious points distributed over the circumference are transmitted fromthe first signal processing unit 9 to the second signal processing unit10. The latter determines, with respect to a number of measuring sectorsof the pipe-wall circumference corresponding to the temperature-controlsectors 12a-12h about the moulding end part 2a of the extrusion die, theaverage of the values measured in said measuring sectors 12a-12b as wellas the average of all values measured over the circumference. Thediagram of FIG. 3 shows the average tube-wall thickness profile permeasuring sector 12a-12h, the sector averages being indicated by S1-S8.

The sector average for each of the measuring sectors 13a-13h is comparedwith the circumferential average, and the value desired of thetemperature of the extrusion-die temperature-control sectorcorresponding the measuring sector concerned of the tube circumferenceis adjusted in the temperature-control unit 8 as determined by thedeviation of the sector average from the circumferential average, in amanner so that the deviation is reduced or eliminated. In the event, forexample that the sector average S1-S8 is lower than the circumferentialaverage, the temperature in the corresponding temperature-control sectorof the moulding end part 2a of the extrusion die 2a is raised, as aresult of which in the location of said temperature-control sector themelt encounters less resistance in the moulding end part 2a of theextrusion die 2, thus locally causing more material to flow through themoulding end part 2a of the extrusion die. Consequently, the wallthickness of the pipe will locally increase.

In this manner, the wall thickness of the pipe 3 at various pointsdistributed over the circumference is controlled as much as possibleaccording to an average value. In conjunction with the control per seknown of the minimum pipe wall thickness by means of the control of thespeed of the drawbench 6, a wall thickness is obtained which is quiteuniform over the circumference and which is very close to the minimumwall thickness desired. It is thus possible to save plastic materialwhile forming a plastic pipe which meets all requirements as to strengthcharacteristics. As it takes some time before a section of the tube 3leaving the moulding end part 2a of the extrusion die 2 reaches thesensing device 7, proper delays have been built into the control.

This also applies to the drawbench-speed control. In the presentembodiment of the system, the wall thickness is measured at 360 pointsuniformly distributed over the circumference, and the wall thicknessaverage is determined for eight measuring sectors distributed over thepipe circumference and each having 45 measuring points. Also, the numberof temperature-control sectors disposed about the moulding end part 2aof the extrusion die is obviously eight. The various signal processingoperations are performed by means of microprocessors. It goes withoutsaying that the heating elements 11 can be used both for heat supply andfor heat removal purposes.

FIG. 5 shows a cross section of the moulding end part 2a of an extrusiondie 2, but now the heating elements 11' are located at the outer side ofthe wall 16 of moulding end part 2a.

EXAMPLE

A polyvinylchloride pipe of 160 mm diameter and 3.2 mm required minimumwall thickness (nominal values) is extruded by means of a two screwextruder without applying the invention.

From measuring data appears that the inner side of the pipe comprisestwo thickened wall zones of 3.7 mm being opposite each other andtherebetween two thin wall zones having a thickness of about 3.4 mm.Such a pipe 3' is shown in FIG. 6a showing the thickened zones 3a and3b.

Use of the process of the invention and measuring the wall thickness at360 points equally distributed over the circumference and the average isdetermined in eight measuring sectors having 45 measuring points persector, provides the same polyvinylchloride pipe 3" (see FIG. 6b)without said thickened wall zones. The pipe presents over the wholecircumference a pipe wall thickness of about 3.3 mm.

It will be obvious that similar results are obtained with thermoplasticpolymers different from polyvinylchloride.

We claim:
 1. A pipe wall thickness controlling unit for use in anapparatus for extruding a solid rigid plastic pipe of a thermoplasticpolymer and controlling the wall thickness of the extruded pipe, saidapparatus comprising a comparator for comparing a wall thicknessmeasuring value with another wall thickness value provided with aconnection to a pipe wall thickness measuring device; a temperaturecontrol unit connected with said comparator and being provided with aconnection to heating elements disposed about the circumference of theextrusion die moulding end part of an extrusion die of an extruder; saidcontrolling unit comprising;a signal processing unit for processing wallthickness measuring data, a comparator element for comparing two valuesof wall thickness measurement value averaging operations, onemeasurement averaging value obtained by averaging data of several wallthickness measurements in one measuring sector, and the othermeasurement averaging value by averaging data of wall thicknessmeasurements in all measuring sectors; a temperature control unitconnected with the comparator element and being provided with aconnection to heating elements disposed in temperature-controlledsectors about the moulding end of an extrusion die of an extruder, saidtemperature control unit being of the type of controlling thetemperature of said heating elements in the absence of cooling means ina temperature-controlled sector located in the axial extension of theone measuring sector aforementioned as a function of the differenceobserved so as to remove the difference measured, such that said heatingelements only effect the temperature of a superficial layer of theplastic pipe body thus reducing or increasing the friction between theouterside of the plastic pipe body and the opposite inner surface of themoulding end part, a too small wall thickness being increased byincreasing the temperature and a too high wall thickness being decreasedby decreasing the temperature in said temperature controlled sectors. 2.A pipe wall thickness controlling unit according to claim 1, saidcontrolling unit being also provided with a connection to drawing meansfor drawing off a plastic pipe body leaving the extrusion die such thatthe difference between the minimum value of the wall thickness asdesired and the minimum value of the wall thickness as measured is usedfor controlling the speed of said drawing means.